The present invention relates to a circuit for estimating the speed of an electromagnetic actuator associated with a reading head of a disk storage unit.
A disk storage device such as a hard disk, CD-ROM or DVD system comprises at least one disk on which data is stored or can be stored, a reading or reading/writing head mounted on a movable arm, and control circuits. The control circuits control an electric motor to rotate the disk about its axis and on an electromagnetic actuator of the type known as a voice coil motor (VCM) for moving the arm radially over the disk so as to position the head over predetermined points on the surface of the disk.
During the normal operation of the device, the head moves over the disk without touching it, supported by air-currents generated by the rotating disk. In some conditions, for example, when reading/writing operations are not to be performed, or when the energy supply to the device is removed, the head is brought to a parked position outside the surface of the disk. This position is typically defined by a ramp onto which the head is loaded (ramp loading), or from which the head is unloaded (ramp unloading).
For head loading operations it is important to apply to the actuator a voltage which is just sufficient to bring the head to the parked position, taking account of the power losses due to friction and leakages. This operation is particularly critical because it is often performed during the switching-off of the device when little electrical energy is available.
The unloading operation, that is, the downward movement of the head from the ramp, is perhaps even more important. In this case, the movement of the head has to be as quick as possible to permit prompt reading/writing of the data, but the head must definitely be prevented from coming into contact with the disk.
To perform the loading and unloading operations correctly, particularly the unloading operation, the speed of the head must be completely controlled in a manner such that the energy supplied to the device is that which is necessary and sufficient for the operation to be performed at every moment.
The instantaneous speed of the head can be estimated by analyzing electrical characteristics of the VCM actuator. The actuator is formed substantially by a coil fixed to the movable arm which carries the head, and by one or more fixed magnets which create a magnetic field that extends through the coil. When a current flows through the coil, a corresponding magnetic force is generated and moves the head relative to the fixed magnet in order to position the head over a predetermined point of the disk.
When the coil cuts the field lines during its movement, a back electromotive force (Vbemf) is generated, which varies based upon the speed of the head. The Vbemf can therefore be measured to make an estimate of the speed. To take a measurement of the Vbemf when a current is flowing through the coil, it is possible to measure the current with a resistor Rs placed in series with the coil, and to take account both of the resistance of the measuring resistor and of the internal resistance Rm of the coil.
According to a known method, the Vbemf is estimated by subtracting the voltage drops in the resistors Rm and Rs from the voltage controlling the VCM actuator. The regulation circuit is calibrated in a manner such that, when the actuator is stationary, the output of the circuit is 0.
A known circuit for controlling a VCM electromagnetic actuator is shown in FIG. 1. A VCM 10 is represented by an inductor Lm (the inductance of the actuator coil), a resistor Rm (the resistance of the coil) and a generator of a voltage Vbemf (the back electromotive force), which are connected in series. The VCM 10 is supplied by a bridge power amplifier represented by two operational amplifiers 11 and 12 with gains of G and -G, respectively. A resistor Rs is connected in series with the VCM 10 to provide a voltage signal proportional to the current passing through the VCM at the input of a measuring amplifier 13 with a gain Gs.
A control voltage Vpxe2x88x92Vm is applied to the bridge amplifier by an error amplifier 14 having a non-inverting input connected to a reference voltage terminal, represented by the ground symbol. An inverting input is connected to a control circuit which supplies it with an analog voltage Vin through a series input resistor R1. The control circuit is formed by a digital controller 15 and by a digital/analog converter 16. The output and the inverting input of the error amplifier 14 are connected to one another by a resistor Rc in series with a compensation capacitor Cc. The output of the measuring amplifier 13 is connected to the inverting input of the error amplifier 14 through a feedback resistor R2.
In operation, a signal containing information relating to the desired position for the head is applied to the error amplifier 14 as a voltage Vin. The bridge amplifier 11, 12 is controlled by the output voltage of the error amplifier 14 and supplies to the VCM 10 a controlled current I which brings the head precisely to the desired position.
The following equation is used to find the speed of the head:
Vpxe2x88x92Vm=Vbemf+(Rs+Rm)*I+L*dI/dt 
With the VCM 10 in a constant current condition, this equation provides:
Vbemf=Vpxe2x88x92Vmxe2x88x92Rs*I*(1+Rm/Rs) 
The Vbemf, and hence the speed of the head, can be estimated precisely if the ratio Rm/Rs is known. The resistance Rs can be determined precisely by selecting a precision resistor which is substantially not sensitive to variations in the operating conditions. However, the resistance Rm, that is, the internal resistance of the VCM, varies with the type of device used at any particular time, and once the actuator has been selected, also varies considerably with the operating temperature (up to 30% by the typical value). The circuit for measuring the Vbemf has to be calibrated accurately to take account of these variations.
A known circuit for estimating the speed of an electromagnetic actuator is shown in FIG. 2. A VCM actuator 10 in series with a measuring resistor Rs are connected between the supply terminals P and M of a bridge power amplifier such as that of FIG. 1, which is not shown in order to not make the drawing unnecessarily complex.
The circuit is formed substantially by an adder, which is formed by an operational amplifier 20 and by a resistive network. More particularly, a resistor 21 with a resistance of R is connected between the terminal P and the non-inverting terminal of the amplifier 20. A resistor 22, also with a resistance of R, is connected in parallel with a resistor 23 of variable resistance Ra to the node S between the resistor Rs and the VCM 10 at one end, and to the inverting terminal of the amplifier 20 at the other end. The output of the amplifier 20 is connected to its inverting input by a resistor 24 with a resistance of Rb.
The non-inverting terminal of the amplifier 20 is also connected, by a variable resistor 25 with a resistance of Raxe2x80x2 to the terminal M, and by a resistor 26 with a resistance of Rb to a source of a constant reference voltage Vref. The output of the amplifier 20 is connected to the digital controller 15 via an analog/digital converter 27. If the variable resistors Ra and Raxe2x80x2 are calibrated correctly, the circuit shown supplies to the controller 15 a signal which depends solely on the Vbemf, and hence on the speed of the head. To calibrate the resistors, the controller 15 measures the voltage between the terminals P and M when the head is stationary, and adjusts the resistors in a manner such that the output of the circuit is at the reference voltage Vref.
The circuit described above can provide precise speed data but has some disadvantages when the circuit has to be integrated in a semiconductor chip with the smallest possible dimensions. The production of the variable resistors Ra and Raxe2x80x2 is in fact quite difficult and expensive in terms of area of the integrated circuit because the calibration range has to be quite large.
In view of the foregoing background, it is therefore an object of the present invention to provide a circuit for estimating the speed of an electromagnetic actuator which does not require an extensive area of the integrated circuit, which is precise, and which is very versatile, i.e., which can be used without modifications, even with electromagnetic actuators which are very different from one another.
This and other objects, advantages and features according to the present invention are provided by a circuit for estimating speed of an electromagnetic actuator associated with a reading head of a disk storage unit and a digital controller. The circuit includes first and second supply terminals and an output terminal, and the first supply terminal is connected to the electromagnetic actuator. A measuring resistor is connected to the second supply terminal, and is connected in series with the electromagnetic actuator for measuring a current which passes therethrough when a supply voltage is applied between the first and second supply terminals.
The circuit preferably further comprises an adder having an output connected to the output terminal for providing an output voltage corresponding to an estimated speed of the electromagnetic actuator, a first input connected to the first supply terminal, and a second input connected to the second supply terminal. A transfer circuit is preferably connected between the first and second supply terminals and the first and second inputs of the adder for transferring to the adder in predetermined voltage proportions and with a predetermined sign the supply voltage and a voltage across the measuring resistor.
The transfer circuit may comprise a calibration circuit connected between the measuring resistor and the second input of the adder. The calibration circuit may be controlled by the digital controller for determining the predetermined voltage proportions so that the output voltage is substantially proportional to a back electromotive force generated by the electromagnetic actuator.
The calibration circuit comprises an adjustable-gain amplifier, and a determination circuit connected to the adjustable-gain amplifier for determining a gain thereof. The determination circuit preferably comprises a coarse setting circuit and a fine calibration circuit controlled by the digital controller.
The coarse setting circuit may comprise a constant reference voltage source, and a resistive voltage divider connected to the constant reference voltage source for deriving a coarse voltage setting. The fine calibration circuit may comprise a plurality of resistors connected in series for defining a plurality of voltage steps for adjusting the coarse voltage setting based upon a digital control signal from the digital controller for generating an adjustable control signal for said adjustable-gain amplifier. The adjusting may comprise at least one of adding and subtracting the voltage steps to the coarse voltage setting.
The determination circuit preferably further comprises a multiplexer connected between the plurality of resistors and the digital controller. The multiplexer has an input receiving the digital control signal and an output providing the adjustment control signal.
Another aspect of the present invention is directed to a method for estimating speed of an electromagnetic actuator using a measuring resistor connected in series therewith. The electromagnetic actuator and the measuring resistor are connected between first and second supply terminals. The method preferably comprises applying a supply voltage across the first and second supply terminals, measuring a current passing through the electromagnetic actuator, and transferring the supply voltage and a voltage across the measuring resistor to an adder for providing an output voltage corresponding to an estimated speed of the electromagnetic actuator.
The transferring may comprise adjusting a gain of an adjustable-gain amplifier connected between the measuring resistor and the adder. The adjusting may comprise deriving a coarse voltage setting, and adjusting the coarse voltage setting based upon a digital control signal for generating an adjustment control signal for adjusting the gain of the adjustable-gain amplifier.